⚛️ phenomenology

New benchmarks for direct detection of freeze-in dark matter in vector portal models

This paper investigates the potential of future direct detection experiments to observe MeV-scale freeze-in dark matter in vector portal models (including dark photons and $U(1)_{L_i-L_j}$ or $U(1)_{B-L}$ extensions), demonstrating that low reheating temperatures enable significant parameter space accessibility via nuclear recoils and solar neutrino signals, even for subcomponent dark matter fractions.

Original authors: David Cerdeño, Patrick Foldenauer, Rafael López Noé, Óscar Zapata

Published 2026-03-18

📖 6 min read🧠 Deep dive

CC BY 4.0

Original authors: David Cerdeño, Patrick Foldenauer, Rafael López Noé, Óscar Zapata

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Big Picture: Hunting the Invisible Ghost

Imagine the universe is filled with a mysterious, invisible substance called Dark Matter. We know it's there because it holds galaxies together with its gravity, but we've never seen it, touched it, or heard it. For decades, scientists have been trying to catch this "ghost" by building massive detectors deep underground.

Most scientists used to think the ghost was a heavy, slow-moving bouncer (called a WIMP) that occasionally bumped into atoms in the detector. But after years of searching, no one has found it.

This paper suggests we might be looking for the wrong kind of ghost. Instead of a heavy bouncer, maybe the ghost is a lightweight, shy teenager (a "MeV-scale" particle) that barely interacts with anything. The authors are asking: If this shy ghost exists, can our next generation of detectors catch it?

The Story: How the Ghost Was Born (The "Freeze-In" Metaphor)

To understand how we might find this ghost, we first need to understand how it was made.

The Old Story (Freeze-Out):
Imagine a crowded party where everyone is dancing and bumping into each other. Suddenly, the music stops, and the lights go out. The dancers stop moving and "freeze" in place. This is how most scientists thought Dark Matter was made: it was hot and active, then the universe cooled down, and the particles got stuck.

The New Story (Freeze-In):
This paper focuses on a different story. Imagine a very shy guest at that party who is so quiet they never talk to anyone. They don't dance; they just stand in the corner. Slowly, over a very long time, a few people from the crowd accidentally bump into them and create a tiny copy of them.

The Catch: This happens so slowly and so rarely that the shy guest never becomes part of the main crowd. They are "frozen in" from the start, never reaching a state of balance with the rest of the universe.
The Result: Because they are so shy, they are incredibly hard to detect.

The Portal: The Vector Mediator

How does this shy ghost talk to us? It needs a bridge. The paper suggests a Vector Portal.

Think of the Standard Model (our known world) and the Dark Sector (the ghost world) as two separate rooms with a locked door. The Vector Mediator is a special key or a messenger that can walk between the rooms.

In some models, this messenger is a Dark Photon (a cousin of the light we see).
In other models, it's a new force carrier that only talks to specific types of particles, like neutrinos (ghostly particles that pass through walls) or electrons.

The Twist: The "Low Reheating Temperature"

Here is the most creative part of the paper. The authors ask: What if the universe didn't get as hot as we thought right after the Big Bang?

Imagine baking a cake.

Standard Scenario: You bake the cake at 350°F. Everything cooks perfectly.
Low Reheating Scenario: You only bake the cake at 200°F. It's much cooler.

If the universe was "cooler" (lower reheating temperature) when the shy ghost was being created:

The "bumping" that creates the ghost happens much less often because there is less energy.
To get the same amount of ghost particles we see today, the ghost must be less shy (it must interact more strongly) to compensate for the cold, low-energy environment.
Why this matters: If the ghost interacts more strongly, it becomes easier to catch in our detectors!

The Hunt: What the Detectors Will See

The paper looks at two ways these detectors might catch the ghost:

1. The "Nuclear Recoil" (The Bowling Ball)
Imagine a bowling ball (the ghost) hitting a pin (an atomic nucleus). If the ghost is heavy enough (between 50 and 500 MeV), it might knock the pin over.

The Good News: In the "Low Reheating" scenario, the ghost is strong enough to knock over the pin. Future detectors like SuperCDMS and DarkSide-20k might finally see this bump.
The Bad News: There is a lot of background noise. Solar neutrinos (tiny particles from the Sun) also hit the pins. This creates a "Neutrino Fog" that can hide the ghost.

2. The "Electron Recoil" (The Ping Pong Ball)
If the ghost is very light, it might hit an electron (a ping pong ball) instead of a nucleus.

The Result: Current detectors (like DAMIC-M and PandaX-4T) have already looked for this. They found that if the ghost is the only dark matter, it's likely already ruled out.
The Loophole: However, if the ghost is only a small fraction (less than 40%) of the total dark matter, it might still be hiding. Future detectors could be sensitive enough to find even a tiny fraction of these ghosts.

The "Double Whammy": Finding the Ghost or the Messenger

The paper points out a fascinating possibility. These models predict two things:

The Ghost: We might see the dark matter particle hitting the detector.
The Messenger: We might see the messenger (the vector mediator) changing how solar neutrinos behave.

Think of it like a crime scene.

Scenario A: You find the thief (Dark Matter).
Scenario B: You don't find the thief, but you find a unique footprint (altered neutrino signals) that proves the thief was there.

In models like $U(1)_{L_\mu - L_\tau}$ (which talks to muons and taus) and $U(1)_{B-L}$ (which talks to protons and neutrons), the "footprint" (neutrino signal) might be so loud that we can detect it even if we miss the thief. This gives scientists a second chance to prove new physics exists.

The Conclusion: Why This Paper Matters

This paper is a roadmap for the next decade of Dark Matter hunting.

It tells us where to look: It suggests that if Dark Matter is light and was born in a "cool" universe, we have a real chance of finding it.
It sets new targets: It tells experimentalists exactly what kind of signals to look for in upcoming detectors like Oscura, TESSERACT, and SuperCDMS.
It offers hope: Even if we don't find the Dark Matter particle itself, we might find the "messenger" or the "footprints" in the neutrino data, proving that the universe is stranger than we thought.

In short: The authors are saying, "Don't give up on the light, shy ghost. If the universe was cooler than we thought, the ghost might be loud enough for our new microphones to finally hear it."

1. Problem Statement

Direct detection experiments have historically focused on Weakly Interacting Massive Particles (WIMPs) produced via thermal freeze-out. However, the lack of WIMP detection has shifted attention to Freeze-In Dark Matter (FIM) scenarios, where Dark Matter (DM) is produced non-thermally through very weak couplings to the Standard Model (SM).

The Challenge: Current experimental constraints (e.g., from DAMIC-M and PandaX-4T) have severely limited the parameter space for minimal Dark Photon models ( $U(1)_D$ ) where DM is a sub-component of the total relic abundance.
The Gap: There is a need to explore alternative vector portal models (specifically anomaly-free $U(1)$ gauge extensions like $U(1)_{L_i-L_j}$ and $U(1)_{B-L}$ ) and non-standard cosmological histories (specifically low reheating temperatures, $T_{rh}$ ) to identify viable regions where FIM DM can still be produced and potentially detected.
The Dual Signature: In these models, new vector mediators also modify neutrino interactions. This creates a dual discovery potential: detecting the DM particle itself or detecting an anomalous flux of solar neutrinos via Coherent Elastic Neutrino-Nucleus Scattering (CE $\nu$ NS).

2. Methodology

The authors employ a comprehensive theoretical and phenomenological framework:

Theoretical Models:
- $U(1)_D$ (Dark Photon): Minimal kinetic mixing between SM hypercharge and a new $U(1)_D$ .
- $U(1)_{L_i-L_j}$ : Anomaly-free gauge groups based on lepton flavor differences (e.g., $L_\mu - L_\tau$ ). These couple directly to neutrinos and specific lepton generations.
- $U(1)_{B-L}$ : Gauge extension of Baryon minus Lepton number, coupling to all quarks and leptons.
- Lagrangian: The study utilizes a general Lagrangian including kinetic mixing terms and mass matrices, diagonalizing them to find physical interactions.
Cosmological Framework (Freeze-In):
- Production Mechanism: DM is produced via SM particle annihilations ( $f\bar{f} \to \chi\chi$ ) and, in specific models, mediator self-annihilations ( $A'A' \to \chi\chi$ ) or neutrino annihilations.
- Low Reheating Temperature ( $T_{rh}$ ): The authors investigate scenarios where the reheating temperature of the universe is lower than the DM mass ( $T_{rh} < m_\chi$ ). This introduces Boltzmann suppression to the production rate, requiring larger gauge couplings ( $g_X$ or $\epsilon$ ) to reproduce the observed relic abundance ( $\Omega_{DM}h^2 \approx 0.12$ ).
- Tools: Numerical calculations are performed using FREEZEIN and micrOMEGAs packages to compute the DM yield ( $Y_\chi$ ).
Direct Detection Analysis:
- Signals: Calculations include both Nuclear Recoils (NR) and Electron Recoils (ER).
- Discovery Limits: The authors perform a maximum likelihood analysis to define two distinct discovery limits:
  1. DM Discovery: Can the DM signal be statistically separated from the SM neutrino background?
  2. Neutrino Discovery: Can the enhanced neutrino spectrum (due to new $A'$ interactions) be distinguished from the SM-only neutrino spectrum?
- Targets: Simulations assume a Silicon (Si) target with a benchmark exposure of 1 tonne-year and varying energy thresholds (1 eV to 100 eV).

3. Key Contributions

Sub-component DM Sensitivity: The paper demonstrates that for ultra-light dark photons, current experiments only constrain DM if it constitutes $>40\%$ of the total relic density. Future experiments (e.g., OSCURA, TESSERACT) could probe DM fractions as low as 1%.
Low Reheating Temperature Impact: The study quantifies how $T_{rh} < m_\chi$ shifts the viable parameter space toward stronger couplings ( $g_X \sim 10^{-2}$ for MeV DM), making these models accessible to direct detection experiments that would otherwise be blind to standard freeze-in scenarios.
Dual Discovery Strategy: The authors establish that direct detection experiments can serve as neutrino observatories. In $U(1)_{L_\mu-L_\tau}$ and $U(1)_{B-L}$ models, the enhanced CE $\nu$ NS cross-section creates a "new physics" signal in the neutrino sector that is distinct from the DM signal, offering a complementary discovery channel.
Model Differentiation: The paper provides specific benchmarks showing how different $U(1)$ models (leptophilic vs. baryonic) yield distinct signatures in NR vs. ER channels, allowing for model discrimination.

4. Key Results

A. Ultra-light Dark Photons ( $U(1)_D$ )

Current Constraints: DAMIC-M and PandaX-4T exclude DM masses $4 \text{ MeV} \lesssim m_\chi \lesssim 500 \text{ MeV}$ for standard cosmology.
Sub-component: If DM is a sub-component ( $\xi < 1$ ), current limits only apply if $\xi > 0.4$ . Future detectors can probe $\xi \sim 0.01$ (1%).
Low $T_{rh}$ : For $T_{rh} \sim 10 \text{ MeV}$ , the parameter space is excluded for $m_\chi > 4 \text{ MeV}$ due to the required increase in coupling to compensate for Boltzmann suppression.

B. Massive Dark Photons ( $U(1)_D$ )

Viable Window: For mediator-to-DM mass ratios $m_{A'}/m_\chi \approx 1/10$ or $1/20$ , a viable window exists for $50 \text{ MeV} \lesssim m_\chi \lesssim 500 \text{ MeV}$ with low $T_{rh}$ .
Detection: In this window, the required couplings are large enough that the DM signal in Nuclear Recoils (NR) can rise above the "neutrino floor" (the background from solar neutrinos), making it detectable by SuperCDMS SNOLAB and DarkSide-20k.

C. Leptophilic Models ( $U(1)_{L_i-L_j}$ )

$U(1)_{L_\mu-L_\tau}$ :
- Strong constraints exist for $m_{A'} \lesssim 10 \text{ MeV}$ from BBN and CMB.
- For $m_{A'} \gtrsim 10 \text{ MeV}$ and low $T_{rh}$ , large parameter spaces are viable.
- NR Sensitivity: Future experiments can detect DM signals for $m_\chi \gtrsim 50 \text{ MeV}$ (depending on energy threshold).
- Neutrino Signal: The enhanced solar neutrino spectrum is distinguishable from SM predictions even if the DM signal is below the threshold, providing a unique "neutrino-only" discovery mode.
$U(1)_{L_e-L_\tau}$ and $U(1)_{L_\mu-L_e}$ :
- These models face much stricter constraints from electron beam dumps and $e^+e^-$ colliders due to coupling to electrons.
- Result: The viable freeze-in parameter space lies below the direct detection sensitivity limits for both NR and ER. Detection relies on mediator searches (e.g., SHiP, NA64) rather than direct DM detection.

D. $U(1)_{B-L}$ Model

Strongest Constraints: Coupling to all hadrons and leptons leads to severe bounds from beam dumps and neutrino experiments.
Best Detection Prospects: Despite constraints, the coupling to hadrons significantly enhances the Nuclear Recoil cross-section.
Result: Large regions of parameter space for $50 \text{ MeV} \lesssim m_\chi \lesssim 500 \text{ MeV}$ with low $T_{rh}$ are above the neutrino floor. Both the DM signal and the enhanced neutrino signal are potentially detectable by future NR experiments.

5. Significance

This paper redefines the search strategy for light Dark Matter in vector portal models:

Shift in Focus: It moves the focus from "standard" freeze-out/thermal freeze-in to low reheating temperature scenarios, which open up new, experimentally accessible parameter spaces with stronger couplings.
Beyond WIMPs: It validates the potential of next-generation direct detection experiments (SuperCDMS, DarkSide-20k, OSCURA) to probe MeV-scale DM that constitutes only a fraction of the total relic density.
Neutrino Physics as a Probe: It highlights that direct detection experiments are uniquely positioned to act as neutrino observatories. The ability to distinguish between a DM signal and a BSM neutrino signal (or detect the latter alone) provides a robust method to identify the underlying particle physics model, specifically distinguishing between leptophilic and baryophilic mediators.
Experimental Guidance: The results provide concrete benchmarks for future experiments, suggesting that lowering energy thresholds (to $\sim 1$ eV) is critical for probing the lower mass end of the viable parameter space, particularly for $U(1)_{L_\mu-L_\tau}$ models.

In conclusion, the authors argue that freeze-in DM in vector portal models, particularly under non-standard cosmological conditions, remains a highly viable and testable hypothesis for upcoming direct detection experiments, offering a dual path to discovery via both Dark Matter and Neutrino sectors.